The present invention relates generally to apparatus for improving the contrast of a displayed image so as to increase the ability of the viewer to discern low contrast objects which are in different backgrounds. The invention may be used in visual or infrared displays and, for example, by pilots or gunners who are attempting to distinguish targets from background areas. It also may be used in facsimile reproduction for automatic thresholding.
Apparatus for detecting or improving video contrast is known in the art. For example, U.S. Pat. No. 3,786,183 shows a system for detecting an area of high contrast for tracking purposes but does not provide for contrast enhancement. U.S. Pat. Nos. 3,952,144 issued to Carl R. Kolker Apr. 20, 1976, and 3,790,706 issued to Thomas J. Jubala Feb. 5, 1974 show devices for controlling contrast but these patents perform only "global" scaling and do not provide for local area enhancement. When global scaling is performed, the entire signal presented to the display is subjected to the same gain. In a scene with large intensity excursions over the entire field of view, this gain is made small so as not to saturate the display and this results in local details being lost. Other prior art such as U.S. Pat. No. 3,181,007 issued to Virgil A. Hinds Apr. 27, 1965, U.S. Pat. No. 3,600,506 issued to William E. Richeson, Jr. Aug. 17, 1971 and U.S. Pat. No. 3,751,586 issued to Bengt H. Johansson Aug. 7, 1973 show circuits in which there is continuous scaling of signals but these patents perform their scaling on a "scan-line" basis wherein each line or a group of entire lines of the video are averaged to compute the background estimate. The difficulty with "scan-line" enhancement is that it does not take into account a local area immediately surrounding every point in the image. When only a fraction of the current line is used in the background average, the estimate of the background does not follow the background changes in the vertical (across the scan-line) direction. When several whole lines are used to compute the average as suggested by the Johnansson U.S. Pat. No. 3,751,586, the estimate is not adaptive to variations of the background in the horizontal (along the scan-line) direction.